The phrase he is most famous for is Cogito Ergo Sum which means "I think therefore I am". Before Descartes, philosophy and theology were pretty much the same thing. But for the first time he didn't start with God in his reasoning, he started with humans. This was quite revolutionary at the time and caused an "explosion" in philosophy and science. We became the center of reasoning instead of god. Which implied there is nothing we won't be able to find out/understand in time.